Semicolons are used to add stronger breaks to sentences than a comma, but less severe than a full stop. Why would you want to do this? Let's look at an example:
The two statements have somewhat different meanings. In the first case, what was rare was the fact that Isobel was crying. It had nothing to do with Gareth!
In the second case, however, the rare event is Gareth being able to sleep whilst Isobel was crying.